Hip dysplasia (HD) in dogs is a developmental orthopedic disease in which abnormal development of the hip leads to functional hip laxity. This results in cartilage degradation, osteophyte formation, subchondral sclerosis, and ultimately osteoarthritis (Henricson B, et al., J Small Anim Pract 1966:673-687; Smith G K., J Am Vet Med Assoc 1997; 210:1451-1457; Riser W H., J Am Vet Rad Soc 1973; 14:35-50; Jones T C, et al., Veterinary pathology. 6th ed. Baltimore: The Williams & Wilkns Co, 1997; 899-946; Morgan J, Wind A, Davidson A. Hip Dysplasia. In: Hereditary bone and joint diseases in the dog: osteochondroses, hip dysplasia, elbow dysplasia. Hannover, Germany: Schlutersche GmbH and Co, 2000; 109-208.). It is empirically believed that fibrotic thickening of the joint capsule associated with osteoarthritis in dogs with HD causes a tightening effect on a hip joint, thereby making the joint more stable (Riser W H., J Am Vet Rad Soc 1973; 14:35-50; Jones T C, et al., Veterinary pathology. 6th ed. Baltimore: The Williams & Wilkns Co, 1997; 899-946; Barr ARS, et al., J Small Anim Pract 1987; 28:243-252). However, in humans, osteoarthritis does not influence subluxation (joint laxity) and will affect position within the hip joint only when the femoral head is malformed (Gofton J P., CMAJ1971; 104:911-915). It is further proposed that tightening and increased stability of the hip joints of affected dogs are associated with less pain. For the most seriously affected dogs, this reduction in pain is usually observed by the time dogs are 18 months old (Barr A R S, et al., J Small Anim Pract 1987; 28:243-252). In addition, it has been theorized that a large active muscle mass may inhibit transformation of passive laxity into functional laxity as a dog ambulates, and thus decrease the stresses on articular cartilage that lead to degenerative changes (Riser W H, et al., Am J Vet Res 1967; 28:769-777; Smith G K, et al., Am J Vet Res 1993; 54:1021-1042).
Currently, there is no cure for osteoarthritis in humans or other mammals. However, there are treatment options for osteoarthritis. For example, non-surgical treatments include use of NSAIDS and nutraceuticals, the modification of nutrition, increase exercise, and physical therapy. Surgery is also an option for end stage disease.
That said, prevention is key in the fight against canine hip dysplasia and osteoarthritis. By predicting the early stages of osteoarthritis, genetic control and selective breeding may be used as an effective means of reducing the severity of canine hip dysplasia and the development of osteoarthritis in subsequent generations of animals by avoiding mating those animals showing signs of osteoarthritis development. However, accurate prediction of osteoarthritis requires a reliable screening method implemented early in life.
Several methods have been proposed to measure passive laxity of the hip joints in humans and dogs. However, two radiographic methods, known as the Norberg angle (NA) and the Distraction index (DI) are used most commonly in the dog (Smith G K, et al., Am J Vet Res 1993; 54:1021-1042; Adams W M, et al., Vet Radiol Ultrasound 2000; 41:484-490; Adams W M, et al., J Am Anim Hosp Assoc 1998; 34:339-347; Owens J, et al., Radiographic interpretation for the small animal clinician. 2nd ed. Baltimore: The Williams & Wilkins Co, 1999; 82-89; Kapatkin A S, et al., J Am Vet Med Assoc 2004; 224:542-546; McLaughlin R Jr, et al., Vet Med 1996; 91:36-47; Lawson D D. Vet Rec 1963; 75:445-456; Lust G L, et al., J Am Vet Med Assoc 2001; 219:1242-1246; Popovitch C A, et al., J Am Vet Med Assoc 1995; 5:648-650; Smith G K, et al., J Am Vet Med Assoc 1990; 196:59-70; Smith G K, et al., J Am Vet Med Assoc 2001; 219:1719-1724; Smith G K, et al., J Am Vet Med Assoc 1995; 206:642-647). The NA measurement method is applied to ventrodorsal hip-extended radiographs as a means of quantifying hip joint laxity (Owens J, et al., Radiographic interpretation for the small animal clinician. 2nd ed. Baltimore: The Williams & Wilkins Co, 1999; 82-89; Olsson S. Adv Small Anim Pract 1961; 3:117-118). Throughout the world, the NA has been incorporated into several screening systems as a measure of joint laxity. However, in the United States, the NA is not measured; rather, joint laxity is subjectively estimated and represented as subluxation.
Another method, known as the PennHIP method (PennHIP, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pa.), consists of three separate radiographs, namely the distraction view, compression view, and ventrodorsal hip-extended view, for full assessment of the spatial relationships of the femoral head to the acetabulum. With the dog on its back and the hip joints in a neutral, stance-phase position, the distraction radiographic view permits quantifying maximum passive hip joint laxity by use of the DI (Smith G K, et al., Am J Vet Res 1993; 54:1021-1042; Adams W M, et al., J Am Anim Hosp Assoc 1998; 34:339-347; Popovitch C A, et al., J Am Vet Med Assoc 1995; 5:648-650; Smith G K, et al., J Am Vet Med Assoc 1990; 196:59-70; Culp W T, et al., Vet Surg 2006; 35:453-459). The compression radiographic view is also obtained with the rear limbs in a neutral, stance-phase position. However, instead of distraction, the femoral heads are compressed into the acetabula to reveal the extent of joint congruency represented by the CI (Smith G K, et al., J Am Vet Med Assoc 1990; 196:59-70). For a perfectly congruent joint, the CI is zero (or within measurement error of zero). Joints that deviate substantially from zero represent grades of incongruency. The ventrodorsal hip-extended view is included in the PennHIP evaluation to help identify key landmarks to aid in accurate measurement of hip laxity appearing on the distraction radiograph and with the new information in the pending patent application, to obtain supplementary information regarding the existence of osteoarthritis in a hip joint as conventionally described. These methods are also described in U.S. Pat. No. 5,482,055, the disclosure of which is incorporated by reference herein as if set forth in its entirety herein.
Irrespective of animal positioning and measurement method or method combinations, no studies have been conducted to examine the change in laxity or congruency of a hip joint associated with the development of radiographic osteoarthritis.
Thus, there is a long felt need in the art for a diagnostic measurement to correlate with degenerative changes associated with early stages of osteoarthritis. The present invention meets this need.